Embodiments herein generally relate to printing systems and methods. More particularly, the embodiments relate to a printing system and method using alternating velocity and torque control modes for operating one or more select sheet transport devices (e.g., a registration nip and/or an image fixing nip) to avoid contention (e.g., with a velocity controlled image transfer nip).
Generally, printing devices incorporate multiple independently driven sheet transport devices (e.g., nips and electrostatic transport belts) for transporting a print media sheet (e.g., a sheet of paper) along a sheet transport path. Oftentimes, these sheet transport devices not only transport a print media sheet but are integral components in other printing operation functions (e.g., sheet registration, image transfer, image fixing, etc.). Thus, velocity matching can become critical in order to avoid image quality disturbances (e.g., shearing, banding, etc.) due to errors, such as registration errors, image on image transfer errors, etc. Current transport device drive schemes typically try to minimize these errors by independently controlling the rotation of each sheet transport device drive roller using a discrete servomechanism. Such a servomechanism monitors sheet velocity (i.e., linear velocity) as a print media sheet is transported by a corresponding sheet transport device and, based on this monitoring, adjusts, with a very tight tolerance, the power supplied to the drive motor which rotates the drive roller in order to achieve a predetermined constant sheet velocity or, alternatively, a predetermined varying sheet velocity profile. In other words, current transport device drive schemes typically operate in a velocity control mode.
Unfortunately, when a single print media sheet is concurrently engaged by multiple independently driven adjacent sheet transport devices (e.g., a registration nip and an image transfer nip, an image transfer nip and an image fixing nip, or a registration nip, an image transfer nip and an image fixing nip), velocity discrepancies between the drive rollers of the devices will cause contention between their corresponding servomechanisms. As a result, a sheet velocity mismatch can occur between the leading edge of a print media sheet and the trailing edge of that same print media sheet at a given point along the sheet transport path. This mismatch can be caused not only by spiking drive roller velocity changes during transitions into and out of contention but also by servomechanisms essentially fighting for velocity control, during contention. Thus, consider a print media sheet being transported along a sheet transport path through a registration nip, a transfer nip and an image fixing nip in series. Since the print media sheet may be engaged by more than one of these nips at a time such that their servomechanisms move in and out of contention, there may be a sheet velocity mismatch between the leading and trailing edges of the print media sheet at the image transfer point. Such a mismatch can result in the same image disturbances (e.g., shearing, banding, etc.) due to the same errors (e.g., registration errors, image on image transfer errors, etc.) that the servomechanisms were designed to avoid.